Various methods of forming ferrite film on a substrate surface have been proposed, which include an application method using a mixture composed of ferrite particles and a binder, and a physical deposition method such as sputtering process. However, a method of growing ferrite crystals on a substrate (hereinafter called "electroless ferrite plating method") has been recently proposed (Japanese Laid-open Patent Application No. 111929/1984). This method is noticeable because an excellent ferrite film with high crystallinity can be formed.
According to the method, as shown in FIG. 9, respective species of ions are absorbed on a substrate as shown in FIG. 9(a) by contacting the substrate with a solution containing ferrous ions (Fe.sup.2+ or FeOH.sup.+) and other metal ions (M.sup.n+ and MOH.sup.n-1+). Although FIG. 9(a) illustrates that individual ions are bonded to oxygen atoms on the substrate, the ions actually are considered to deposit on the substrate by various reasons such as binding with oxygen or absorption. Afterwards, the ions formed on the substrate are oxidized as shown in FIG. 9(b). The oxidized ions react to form a ferrite film as illustrated in FIG. 9(c). Subsequently, the former condition shown in FIG. 9(a) resumes. Ferrite films successively grow with the recurrence of above mentioned steps.
The electroless ferrite plating method is highly rated as an excellent technique to form a ferrite film on a plate-like substance such as a magnetic tape or disk.
However, every application of ferrite films formed by the electroless ferrite plating method is exclusively associated with a plate-like substance, and particles or fibrous substances has never been considered as a substrate for the electroless ferrite plating method. In the electroless ferrite plating method, it is believed that the ferrite forming reaction not only occurs on particulate or fibrous substrates as shown in FIG. 9, but also occurs in the solution to by-produce ferrite particles. Thus, it is difficult to separate the resultant product from the by-producted ferrite particles. Even when forming a ferrite film on a plate-like substance, inhibiting the accompanying generation of particle ferrite is a vital requirement concerning quality and other aspects. Due to the above reasons, application of the electroless ferrite plating method to particulate substrates has been considered to be impossible.